Indonesian Earthquakes Set Record!

A pair of massive earthquakes that occurred earlier today off the coast of Indonesia, reminiscent of the tragic magnitude 9.1 quake that struck in 2004, caused a fair bit of panic but ultimately minimal damage. The earthquakes were extremely unusual, setting a new record for strike-slip faults—vertical faults where the blocks slide horizontally past each other. Most major earthquakes occur on ramp-like thrust faults associated with subduction zones, where one block is being pushed up over another.

Previously, the largest earthquakes recorded on strike-slip faults included the 1906 Great San Francisco Earthquake at roughly magnitude 7.9 and the 1855 Wairarapa earthquake in New Zealand at around 8.2. The first quake in Indonesia today was a whopping magnitude 8.6 centered about 200 miles west of the 2004 earthquake. A magnitude 8.2 aftershock followed a couple hours later another 100 miles southwest from the first. Since the movement in each earthquake was horizontal rather than vertical, no significant tsunami waves were produced.

The faults involved are related to a diffuse (that is, “fuzzy”) plate boundary between the Indian and Australian plates. A few strike-slip earthquakes have been detected there before, but they were small and scattered enough that no major fault was identified. There were certainly no indications that the boundary might produce a record-breaker.